linux-kernel - Re: [PATCH 3/4] KVM: nVMX: Track whether changes in L0 require MSR bitmap for L2 to be rebuilt

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Date:   Mon, 13 Sep 2021 08:57:49 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] KVM: nVMX: Track whether changes in L0 require MSR
 bitmap for L2 to be rebuilt

Maxim Levitsky <mlevitsk@...hat.com> writes:

> On Fri, 2021-09-10 at 18:06 +0200, Vitaly Kuznetsov wrote:
>> Introduce a flag to keep track of whether MSR bitmap for L2 needs to be
>> rebuilt due to changes in MSR bitmap for L1 or switching to a different
>> L2. This information will be used for Enlightened MSR Bitmap feature for
>> Hyper-V guests.
>> 
>> Note, setting msr_bitmap_changed to 'true' from set_current_vmptr() is
>> not really needed for Enlightened MSR Bitmap as the feature can only
>> be used in conjunction with Enlightened VMCS but let's keep tracking
>> information complete, it's cheap and in the future similar PV feature can
>> easily be implemented for KVM on KVM too.
>> 
>> No functional change intended.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  arch/x86/kvm/vmx/nested.c | 9 ++++++++-
>>  arch/x86/kvm/vmx/vmx.c    | 2 ++
>>  arch/x86/kvm/vmx/vmx.h    | 6 ++++++
>>  3 files changed, 16 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
>> index ccb03d69546c..42cd95611892 100644
>> --- a/arch/x86/kvm/vmx/nested.c
>> +++ b/arch/x86/kvm/vmx/nested.c
>> @@ -2053,10 +2053,13 @@ static enum nested_evmptrld_status nested_vmx_handle_enlightened_vmptrld(
>>  	 * Clean fields data can't be used on VMLAUNCH and when we switch
>>  	 * between different L2 guests as KVM keeps a single VMCS12 per L1.
>>  	 */
>> -	if (from_launch || evmcs_gpa_changed)
>> +	if (from_launch || evmcs_gpa_changed) {
>>  		vmx->nested.hv_evmcs->hv_clean_fields &=
>>  			~HV_VMX_ENLIGHTENED_CLEAN_FIELD_ALL;
>>  
>> +		vmx->nested.msr_bitmap_changed = true;
>> +	}
>> +
>>  	return EVMPTRLD_SUCCEEDED;
>>  }
>>  
>> @@ -3240,6 +3243,8 @@ static bool nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
>>  	else
>>  		exec_controls_clearbit(vmx, CPU_BASED_USE_MSR_BITMAPS);
>>  
>> +	vmx->nested.msr_bitmap_changed = false;
>
> Very minor nitpick: Maybe I would put this into nested_vmx_prepare_msr_bitmap,
> a bit closer to the action, but this is fine like this as well.
>

I don't have a strong preference here, can move it to nested_vmx_prepare_msr_bitmap().

>> +
>>  	return true;
>>  }
>>  
>> @@ -5273,6 +5278,7 @@ static void set_current_vmptr(struct vcpu_vmx *vmx, gpa_t vmptr)
>>  		vmx->nested.need_vmcs12_to_shadow_sync = true;
>>  	}
>>  	vmx->nested.dirty_vmcs12 = true;
>> +	vmx->nested.msr_bitmap_changed = true;
>>  }
>>  
>>  /* Emulate the VMPTRLD instruction */
>> @@ -6393,6 +6399,7 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
>>  		goto error_guest_mode;
>>  
>>  	vmx->nested.dirty_vmcs12 = true;
>> +	vmx->nested.msr_bitmap_changed = true;
>
> Is this needed? Setting the nested state should eventually trigger call to
> nested_vmx_handle_enlightened_vmptrld.
>
>  

Strictly speaking - no (meaning that nothing is going to change if we
just drop this hunk). My intention was to keep tracking information
complete: after vmx_set_nested_state() we certainly need to re-build MSR
Bitmap 02 and that's what 'msr_bitmap_changed' tracks. We can replace
this with a comment if needed (but I'd slightly prefer to keep it -
unless there's a reason not to).

>
>>  	ret = nested_vmx_enter_non_root_mode(vcpu, false);
>>  	if (ret)
>>  		goto error_guest_mode;
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index ad33032e8588..2dbfb5d838db 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -3734,6 +3734,8 @@ static void vmx_msr_bitmap_l01_changed(struct vcpu_vmx *vmx)
>>  	 */
>>  	if (static_branch_unlikely(&enable_evmcs))
>>  		evmcs_touch_msr_bitmap();
>> +
>> +	vmx->nested.msr_bitmap_changed = true;
>>  }
>>  
>>  void vmx_disable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type)
>> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
>> index 4858c5fd95f2..b6596fc2943a 100644
>> --- a/arch/x86/kvm/vmx/vmx.h
>> +++ b/arch/x86/kvm/vmx/vmx.h
>> @@ -148,6 +148,12 @@ struct nested_vmx {
>>  	bool need_vmcs12_to_shadow_sync;
>>  	bool dirty_vmcs12;
>>  
>> +	/*
>> +	 * Indicates whether MSR bitmap for L2 needs to be rebuilt due to
>> +	 * changes in MSR bitmap for L1 or switching to a different L2.
>> +	 */
>> +	bool msr_bitmap_changed;
>> +
>>  	/*
>>  	 * Indicates lazily loaded guest state has not yet been decached from
>>  	 * vmcs02.
>
>
> Best regards,
> 	Maxim Levitsky
>

-- 
Vitaly